In many applications it is important to differentiate between the bulk properties of a material or device and the surface properties of that material or device. The bulk or substrate material provides a set of bulk properties suitable for the intended application, such as mechanical properties or refractive properties. However, in many applications the surface properties of the substrate material are not suitable or ideal for the intended application. Accordingly, for these substrate materials surface modification is required to mask the surface properties of the substrate that may interfere with use of the substrate.
For example, surface modification to mask the properties of a bulk material is useful in biomedical applications. The bulk material of an implant, such as a polymeric corneal onlay or metal hip implant, is selected for refractive and/or mechanical properties. However, the surface properties of the bulk material may interfere with a desired biological response, such as cell attachment, or may provoke an undesired biological response, such as protein fouling. Furthermore, surface modification may be used to achieve a degree of control over the biological response to the material or device that cannot be achieved by the properties of the bulk material itself. Examples of ways one can achieve control over the biological response include the display of specific bioactive signals such as peptides or drugs on the surface of a device or the prevention of non-specific interactions between the surface of the device and the surrounding biological medium.
One surface modification technique developed to mask the surface properties of bulk materials is the immobilisation of polymers on a substrate surface. This has generally been achieved, with varying degrees of success and completeness, by adsorption or covalent bonding.
Adsorption is the simplest method of immobilising macromolecules onto a surface. However, adsorbed coatings may desorb under certain conditions, which limits the appropriateness of adsorption as a surface modification technique in a variety of biomedical applications. For example, devices designed to remain in physiological conditions for an extended period of time, such as implants, may lose their functional surface coating, leaching polymers into their environment. Furthermore, simple adsorption is not readily applicable to a large variety of substrate materials, as the properties of the substrate are not suitable for attracting and retaining the coating polymers. Whether the properties of a substrate are appropriate will also depend on the type of polymer that is to be applied.
WO 03/042724 describes a more sophisticated process for coating a material surface based on adsorption, comprising the steps of: a) providing an inorganic or organic bulk material; b) providing one or more polyionic molecules with at least one of them having pendant covalently bound initiator moieties for radical polymerization; c) applying the polyionic material of step (b) to the bulk material of step (a), thereby forming a hydrophilic layer on the bulk material surface; and d) graft polymerizing a hydrophilic monomer or macromonomer to said polyionic material. Disadvantages of this approach include the possibility of desorption of the coating as described for the adsorbed surface modification techniques. Furthermore, control over the grafted polymer coating architecture is limited.
Alternatively, polymers are immobilised onto the surface of a substrate by covalent bonding. A variety of different processes for obtaining graft polymer coatings on solid substrates have been described. Some examples of this technique include:
1) EP 1 095 711 A2 describes the covalent attachment to a substrate of conventional radical polymerisation initiators which may be used for the coating of biomedical substrate materials. This approach typically leads to an indeterminate and inconsistent density of the initiator across the surface of a substrate, which is often undesirable. Furthermore, disadvantageously the initiator density that can be achieved with this procedure is limited. Finally, there is only limited control over the subsequently grafted coating architecture.
2) Initiator-containing self-assembled monolayers have also been used. For example, Boven et al. [Boven, G., Folkersma, R., Challa, G., Schouten, A. J., Polym. Commun. 32 (1991) 50] treated glass beads with 3-aminopropyltriethoxysilane to obtain amino functional groups on the surface. Azo-initiators were then immobilised on the surface through the formation of amide bonds between the g-APS modified surface and an acid chloride functionalized azo-initiator. Subsequent surface initiated radical polymerisation produced tethered PMMA chains. Disadvantages of this approach again include an indeterminate, inconsistent, and limited density of the initiator across the surface of a substrate and limited control over the grafted coating architecture. Furthermore, this approach uses a multi-step surface coating approach which limits its applicability and usefulness to silica-type materials.
3) Sugawara, T., Matsuda, T., Macromolecules 27 (1994) 7809 describes grafting poly(acrylamide) on poly(ethylene terephthalate) (PET) substrates. First the substrate is coated with poly(allylamine), which had been partially derivatised with photoreactive phenylazido groups. The aminated polymer was then bound to the surface of the PET substrate by UV irradiation. Carboxylated azo-initiators were then immobilized on the polyamine modified surface through a condensation reaction. Radical polymerisation in a monomer solution finally yielded tethered polymers. Again, this technique uses a multi-step surface coating approach which limits its applicability. Other disadvantages of this approach again include an indeterminate, inconsistent and limited density of the initiator across the surface of a substrate and limited control over the grafted coating architecture.
4) Graft polymerisation initiators have also been immobilised on solid substrates by simple swelling in a solution containing the initiator and subsequent graft polymerisation in a solution containing the initiator. U.S. Pat. No. 6,358,557 discloses this concept. Furthermore, WO 03/083040 teaches the use of primer layers incorporating the initiator. This method can also be used for solid substrates that are incapable of swelling. Apart from substrate dependency issues, disadvantages of this approach again include the indeterminate, inconsistent and limited density of the initiator across the surface of a substrate and limited control over the grafted coating architecture. Furthermore, the lack of covalent attachment can lead to partial desorption of the coating.
ATRP initiators have been covalently attached to silica surfaces via a silane reaction with surface hydroxyl groups and to gold surfaces by the reaction of ATRP initiators containing thiol groups with the gold surface [Pyun, J., Kowalewski, T. Matyjaszewski, K., Macromolecular Rapid Communications, 24 (2003) 1043]. However, reliance on the use of substrates such as silica and gold limits the applicability of this technology. In addition, further disadvantages include, that self assembled layers formed from silanes have been shown to be somewhat unstable [Wang, A. et al., Journal of Colloid and Interface Science, 291 (2005) 438] and irreproducible [Halliwell, C. M., Cass, A. E. G., Analytical Chemistry 73 (2001) 2476] and that self assembled layers formed between gold and thiols (a non-covalent interaction) have been shown to be unstable with time [Willey, T. M. et al., Surface Science 576 (2005) 188]. Furthermore, the formation of such modified surfaces is relatively complex (with the requirement that the substrate be scrupulously clean and dry) and the surface coating may not be evenly distributed on the substrate. The covalent attachment of other initiators, such as iniferters, has also been used to form graft polymer layers on substrates such as silica (via silanes) [Lee, H. J., Nakayama, Y., Masuda, T., Macromolecules 32 (1999) 6989] and polystyrene via derivatisation reactions [Nakayama, Y., Matsuda, T., Langmuir 15 (1999) 5560; Kawaguchi, H., (sono, Y. Tsugi, S., Macromolecular Symposia 179 (2002) 75]. However, as previously discussed, surface modification schemes designed for particular substrates, such as silica and polystyrene, severely limits the applicability of the technology.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood or regarded as relevant by a person skilled in the art.